A spokesman for Buckingham Palace said the queen was not inside at the time and declined to comment further, saying it was a matter for the police.

The palace, which is open only at limited times to public visitors, was not open to the public on Monday.

A month ago, police arrested two men over a break-in at the palace in one of the most serious security breaches there in around 30 years.

One man was found inside Buckingham Palace and was arrested for burglary, trespass and criminal damage and a second was arrested outside on suspicion of conspiracy to commit burglary. Both were bailed and are due back before police this week.

Two days later, Queen Elizabeth's son Prince Andrew was confronted by armed police in the gardens of Buckingham Palace and asked to verify his identity. Police later apologized for failing to recognize the prince.

A man with a knife at Buckingham Palace was arrested and charged with trespassing and possession of a bladed article, just a month after another security incident at the London residence of Queen Elizabeth.